A key component in many architectures of high speed, optical signal distribution systems is a single mode fiber passive star coupler. Under ideal conditions, a star coupler having N inputs and N outputs, hereinafter referred to as an N.times.N star, divides the power entering any of the N input ports equally between all of the N output ports. Naturally, there is some loss of power within the star coupler due to absorption and scattering. The N.times.N star coupler can be used as the central node of a high speed, optical, local-area network where each user can be connected by two fibers-one for transmission to the input side of the star, and the other for reception from the output side of the star. This arrangement creates a broadcast type of local area network where a message which is transmitted by any user can be received by all users. In use, different messages can be transmitted and received simultaneously by the various users through the application of any of a number of protocols such as wavelength division multiple access or time division multiple access.
A simple 2.times.2 star coupler is a 3 db coupler which comprises two fibers which share their optical power at a point of proximity. Such structures are made by etching, grinding and polishing, or fusion techniques. Such 2.times.2 stars can be ganged together to provide larger N.times.N stars, where N is any arbitrary power of two. When multimode waveguides are used, the star coupler can be a glass element with each subscriber's output connected at one side of the element, and each subscriber's access line connected to the other side of the element.
In one prior art arrangement disclosed in U.S. Pat. No. 4,904,042, the coupler input and output waveguide facets are arranged in circular, input and output arrays. The center of curvature of the circle segment which defines the input array facets lies on the circle segment which defines the output array facets, and vice versa. The axes of the waveguides in each array are directed to the center of curvature of the circle defining the configuration of that array. The radius of the circles defining the configuration of each array is selected to maximize the transmission from the marginal input element to the marginal output elements. The aperture of each of the elements may be further varied from element to element to maximize the efficiency of the array.
In the arrangement of U.S. Pat. No. 4,904,042, the input and output array lie in a single plane. Thus, the number of waveguides in the input and output arrays are limited to the number of waveguides that can be positioned side-by-side in the circular array.
A major disadvantage of the star coupler system is associated with the fact that the signal from each input waveguide must be distributed among all of the output waveguides. Accordingly, power considerations are important as the power from each input waveguide is divided among all of the output waveguides. Additionally, single mode star couplers having multitudes of input waveguides and multitudes of output waveguides require large numbers of ganged, stacked or cascaded directional couplers. Such couplers are not only extremely difficult to fabricate, but are economically prohibitive and relatively large in size.